a) Field of the Invention
The present invention relates to a charge transfer device, a solid state image pickup device, and a control method for a solid state image pickup device.
b) Description of the Related Art
FIGS. 15A and 15B show an example of a general charge transfer device. FIG. 15A is a plan view and FIG. 15B is a schematic cross sectional view.
As shown in FIG. 15B, a charge transfer device X has: a semiconductor substrate 101 having a principal surface layer of a p-type or an i-type; an n-type semiconductor layer 105 formed on the surface of the semiconductor substrate 101; charge transfer electrodes 121 of polysilicon formed on an insulating film 115 on the n-type semiconductor layer 105; and a pulse signal generator 125 for applying pulse voltages to the charge transfer electrodes.
The charge transfer electrodes 121 include first charge transfer electrodes 121-1, 121-3, 121-5, . . . made of first layer polysilicon and second charge transfer electrodes 121-2, 121-4, 121-6, . . . made of second layer polysilicon. The first and second charge transfer electrodes are disposed alternately in a horizontal direction.
The regions of the n-type semiconductor layer 105 under the first charge transfer electrodes 121-1, 121-3, 121-5, . . . made of the first layer polysilicon have a low n-type impurity concentration, forming potential barriers (B). The regions of the n-type semiconductor layer 105 under the second charge transfer electrodes 121-2, 121-4, 121-6, . . . made of the second layer polysilicon have a high n-type impurity concentration, forming potential wells (W).
First and second two charge transfer electrodes 121 adjacent to each other in the horizontal direction, e.g., the first and second charge transfer electrodes 121-1 and 121-2, or 121-3 and 121-4, are connected in common. A pulse signal Φ2 is applied from the pulse signal generator 125 to the commonly connected charge transfer electrodes 121-1 and 121-2. A pulse signal Φ1 is applied from the pulse signal generator 125 to the commonly connected charge transfer electrodes 121-3 and 121-4.
Similarly, pairs of first and second two adjacent charge transfer electrodes disposed alternately in the horizontal direction are alternately applied with pulse signals Φ2, Φ1, Φ2, Φ1, . . . from the pulse signal generator 125.
FIG. 16 shows the waveforms of pulse signals generated from the pulse signal generator 125 (FIG. 15B).
The pulse signal Φ1 has a waveform with a low level and a high level being set alternately. The pulse signal Φ2 has a waveform opposite in phase to the pulse signal Φ1.
As the pulse signals Φ1 and Φ2 are applied to the charge transfer device X shown in FIGS. 15A and 15B, charges are transferred in the horizontal direction by a two-phase drive method as shown in FIG. 15B.
In addition to the two-phase drive method, charges may be transferred by a three-phase drive method, a four-phase drive method or the like.
In the charge transfer device of a two-phase drive type, a pair of first and second two charge transfer electrodes 121 constitutes one charge transfer stage. In the charge transfer device of the two-phase drive type described above, each charge transfer stage is fixed.
FIG. 17 is a plan view showing a solid state image pickup device A using the charge transfer device X shown in FIGS. 15A and 15B.
The solid state image pickup device A shown in FIG. 17 has: a plurality of photoelectric conversion elements 103 disposed regularly on a two-dimensional plane of the surface of a semiconductor substrate 101; a plurality of vertical charger transfer paths 105 for reading signal charges accumulated in the photoelectric conversion elements 103 and sequentially transferring the read charges in a column direction; a horizontal charge transfer path 107 connected to one ends of the vertical charge transfer paths 105 for transferring charges transferred from the vertical charge transfer paths 105 in the horizontal direction; and an output amplifier 111 for amplifying the charges transferred from the horizontal charge transfer path 107 and outputting the amplified charges.
A read gate 103a is formed between the photoelectric conversion element 103 and vertical charge transfer path 105 to read charges accumulated in the photoelectric conversion element 103 to the vertical charge transfer path 105. Charges are transferred in the vertical charge transfer path 105 toward the horizontal charge transfer path 107.
The charge transfer device X is used as the horizontal charge transfer path 107.
One end of the vertical charge transfer path 105 is connected to the high concentration n-type semiconductor layer 105a forming a well layer in the horizontal charge transfer path 107.
The two-phase drive type charge transfer device described above is driven by two-phase drive pulses. Conventional three-phase or four-phase drive type charge transfer devices not using barrier layers and well layers transfer charges by three-phase or four-phase drive pulses, and cannot transfer charges by different methods.
For example, in the two-phase drive type charge transfer device, two charge transfer electrodes constitute one charge transfer stage and a geometrical length of one charge transfer stage is fixed. Such constriction poses some problem when the charge transfer device is used as the horizontal charge transfer path of a solid state image pickup device. Specifically, in a solid state image pickup device, one charge transfer stage of the horizontal charge transfer path is provided for each vertical charge transfer path in order to transfer all image signals from the vertical charge transfer paths to the horizontal charge transfer path.
In an image monitor mode, it is desired to reproduce an image at high speed by thinning the number of read pixels. It is also desired to thin the numbers of both rows and columns. In this case, if signal charges (electrons) are transferred only in even columns, signal charges are stored in the transfer stages of every second even columns, and charges are not stored in the transfer stages in odd columns adjacent in the horizontal direction so that the transfer stages are empty. This horizontal charge transfer path processes both an image signal and an empty signal, and transfers even an empty charge as a signal.
Therefore, even if the number of signal bits is reduced, a substantial data rate of the horizontal charge transfer path of a solid state image pickup device does not lower nor a consumption power reduces.